1. Field of Invention
The present invention concerns zinc-free glass frits that can be used to formulate ceramic glaze compositions that develop glossy surfaces when fired on ceramic products such as architectural tile.
2. Description of Related Art
Ceramic products such as architectural tiles, for example, are formed from batches of natural or synthetic raw materials such as clay, kaolin, quartz, feldspar, wollastonite, talc, calcium carbonate, dolomite, calcined kaolin, hard clay, chamotte-grog, oxides such as alumina, silica, corundum and mixtures of the foregoing. Binders and other additives may also be employed to increase the mechanical strength of the raw (“unfired”) ceramic bodies. Once formed, the raw ceramic bodies must be fired to form biscuits that are hard, tough, and brittle.
Glaze compositions, which typically comprise one or more glass frits and other optional raw materials, pigments and additives, are often applied to and fired on the ceramic bodies to impart a protective surface to the ceramic product. A variety of glazing and firing methods are known in the art, including a single fire approach in which the glaze composition and the raw ceramic body are fired simultaneously to form a fired ceramic product having a protective vitreous surface layer bonded thereto. In addition to the single fire approach, it is also well known in the art to produce glazed ceramic tiles by a double fire approach. In the double fire approach, the glaze composition is applied to a once-fired biscuit, which is then subjected to a second firing operation in order to produce the glazed ceramic product.